The present disclosure relates generally to a flowmeter, particularly to an ultrasonic flowmeter, and more particularly to an ultrasonic flow meter with acoustic noise attenuating features integrally formed with a one-piece conduit of the flowmeter.
Ultrasonic flowmeters use ultrasonic techniques, such as ultrasonic transit time techniques, ultrasonic phase shift techniques, and/or ultrasonic sing-around techniques, for example, to measure fluid flow and take other fluid measurements, including flow velocity, flow rate, Reynolds Number, and acoustic speed of the fluid, for example. Ultrasonic flowmeters using ultrasonic techniques may be used for blood flow measurement in an extracorporeal tube, or water flow in plastic, glass or steel pipes, for example. Such fluid flow measurements can be employed for all fluids, including liquids and gases, as well as suspensions, slurries, and multiphase flowable materials.
There are generally two types of ultrasonic flowmeters, which are herein referred to as type-1 and type-2 flowmeters. In a type-1 flowmeter 100 as depicted in FIG. 1, one pair or several pairs of ultrasonic transducers 104, 106 are clamped, installed or fitted on a flowmeter body 102, which is usually composed of a large pipe disposed between connecting structures on each end, such as a flange or a pipe fitting. In the embodiment of FIG. 1, pipe clamps 108 (with adjustment screws hidden from view) may be used to hold the transducers 104, 106 in acoustic signal communication with the body 102 by engaging flanges 110 of mounting blocks 112, to which the transducers 104, 106 are fitted, thereby securing the flanges 110 and mounting blocks 112, along with the transducers 104, 106, to the body 102. By loosening the pipe clamps 108, an operator in the field can adjust the axial and circumferential positioning of the transducers 104, 106 relative to the body 102. The ultrasonic transducers may have piezoelectric crystals in the transducer housings to transmit and receive ultrasonic signals. The ultrasonic transducers can be capacitive micro-machined ultrasonic transducers (CMUTs), piezocomposites, or other piezoelectric materials. In an embodiment the transducers 104, 106 of the type-1 flowmeter 100 are mounted in pairs on the flowmeter body 102 opposite each other, one upstream and another downstream. The paired transducers 104, 106 transmit and receive ultrasonic signals alternately in a diagonal manner through the fluid in the pipe. The differences in ultrasound propagation time or ultrasonic wave phases between upstream and downstream received signals are proportional to the flow velocity.
Liquid flows through the pipe body 102 and the flow characteristics and fluid properties are measured by exciting the transducers and comparing the time differences or phase shifts between received signals. The transducers are typically attached to the coupling wedges permanently and the coupling block will direct the ultrasonic waves through the pipe walls and the fluid. Coupling materials like gel or grease can be applied between the coupling wedge and the pipe wall to improve the signal transmission. In certain applications, instead of clamp-on transducers, the ultrasonic transducer can be permanently mounted in the pipe wall or a “wetted” transducer mounted through the pipe wall can be used to improve the signal strength.
In a type-2 flowmeter 200 as depicted in FIG. 2, a pair of ultrasonic transducers 204, 206 are clamped, installed or fitted on respective ends of a “U-shaped” or “Z-shaped” tube section 202. FIG. 2 shows a “U-shaped” tube section where the legs 201 may be straight (depicted by solid lines) or angled (depicted by dashed lines), and FIG. 3 shows a “Z-shaped” tube section where the legs 203 may be straight (depicted by solid lines) or angled (depicted by dashed lines). In an embodiment the transducers can be acoustically coupled to the ends of the measurement section, or they can have direct contact with the fluid, which is referred to as a wetted transducer. Operation of the type-2 flowmeter 200 is similar to that of the type-1 flowmeter 100. The geometry of a type-2 flowmeter 200 may cause a slight pressure drop at the location where the flow direction is changed.
Accurate measurement of transit time differences or phase shifts depends to a large degree on the signal to noise ratio (SNR) of the downstream and upstream received ultrasonic signals. The higher the SNR, the better the accuracy that can be achieved. Multiple methods have been employed in prior art devices to improve the SNR, such as increasing the driving voltage on the transmitter and adding filtering circuits to the receiver, for example. These methods help distinguish the ultrasonic signal that carries the flow information from random background noise.
In some situations, the ultrasound wave propagates not only through the fluid but also through the conduit wall. If the conduit portion of the ultrasound wave arrives at the receiving transducer close to the same time as the fluid portion of the ultrasound wave, the conduit portion of the ultrasonic wave may interfere with the fluid portion of the ultrasonic wave, which carries the transit time and/or phase difference information of interest, increasing the magnitude of errors in flow measurements of the fluid itself.
U.S. Pat. No. 5,969,263 discloses the placement of absorbing materials inside a gas flow chamber to absorb or attenuate the parasitic ultrasonic noise generated by pressure regulators. However, this method requires the addition of such absorbing materials into the flowmeter, which is not feasible for most non-invasive measurements and liquid flowmeters.
U.S. Pat. No. 6,490,933 discloses the external attachment of a separate acoustic filter to a measurement tube to cut high frequency noises propagating through the oscillating tube. Such external attachments are not desirable in many applications, as they require more space and assembly, and they introduce a discontinuous boundary layer between the fluid conduit and the externally attached noise attenuating features that may deflect acoustic waves back into the conduit wall thereby reducing the overall efficiency of the noise attenuating features. Furthermore, a separately attached acoustic filter is typically designed as a frequency cut-off filter having filtering characteristics that must be matched to the vibrational characteristics of the structure of the oscillating tube that it is attached to.
U.S. Pat. No. 7,624,651 discloses an apparatus for attenuating acoustic waves in a pipe wall for a clamp-on ultrasonic flowmeter that employs a housing mounted to the flow pipe, the mounted housing providing multiple impedance changes, via viscoelastic damping material embedded within slots in the housing, which serves to dissipate vibrational energy in the pipe wall. Such external attachments are not desirable in many applications, as they require more space and assembly, and they introduce a discontinuous boundary layer between the fluid conduit and the externally attached noise attenuating features that may deflect acoustic waves back into the conduit wall thereby reducing the overall efficiency of the noise attenuating features. Furthermore, a separately mounted damping housing is typically designed to match the vibrational characteristics of the structure of the flow pipe that it is attached to.
U.S. Pat. No. 7,963,177 discloses an apparatus for attenuating ultrasonic waves propagating within a pipe wall of an ultrasonic flowmeter that employs a clamped-on damping device having tines that are mounted to panels where the tines are tuned to avoid frequency resonance and to dissipate high frequency energy in the pipe wall. Such external attachments are not desirable in many applications, as they require more space and assembly, and they introduce a discontinuous boundary layer between the fluid conduit and the externally attached noise attenuating features that may deflect acoustic waves back into the conduit wall thereby reducing the overall efficiency of the noise attenuating features. Furthermore, a separately attached acoustically tuned damping device is typically designed as a frequency resonance or frequency cut-off filter having filtering characteristics that must be matched to the vibrational characteristics of the structure of the pipe that it is attached to.
While existing ultrasonic flowmeter damping devices may be suitable for their intended purpose, the art of ultrasonic flowmeters can be advanced with an improved apparatus that reduces the parasitic ultrasonic noise propagating through the ultrasonic flowmeter conduit wall without the foregoing disadvantages.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.